Fuel cell systems, such as fuel cell based power plants and mobile fuel cell based power generation equipment, generate electrical power via electrochemical reactions, and are coming into greater use because the exhaust byproducts are typically cleaner than traditional power plants, and because fuel cells may generate electricity more efficiently than traditional power plants. Fuel cell systems often employ stacks of individual fuel cells, each fuel cell typically including an anode, a cathode, and an electrolyte positioned between the anode and the cathode. The electrical load is coupled to the anode and the cathode. The anode and cathode are electrically conductive and permeable to the requisite gases, such as hydrogen and oxygen, respectively. In a solid oxide fuel cell (SOFC), the electrolyte is configured to pass oxygen ions, and has little or no electrical conductivity so as to prevent the passage of free electrons from the cathode to the anode. In order for the electrochemical reactions to take place efficiently, some fuel cells are operated at elevated temperatures, e.g., with anode, cathode and electrolyte temperatures in the vicinity of 700° C. to 1000° C. or greater for an SOFC.
During normal operation, a synthesis gas is supplied to the anode, and an oxidant, such as air, is supplied to the cathode. Some fuel cell systems include an internal reformer that catalytically reforms the fuel into a synthesis gas (syngas) by use of an oxidant. The fuel may be a conventional fuel, such as natural gas, gasoline, diesel fuel, or an alternative fuel, such as bio-gas, etc. The synthesis gas typically includes hydrogen (H2), which is a gas frequently used in fuel cells of many types. The synthesis gas may contain other gases suitable as a fuel, such as carbon monoxide (CO), which serves as a reactant for some fuel cell types, e.g., SOFC fuel cells, although carbon monoxide may be detrimental to other fuel cell types, such as PEM (proton exchange membrane) fuel cells. In addition, the synthesis gas typically includes other reformer byproducts, such as water vapor and other gases, e.g., nitrogen and carbon dioxide (CO2), methane (typically 1%), as well as trace amounts of higher hydrocarbon slip, such as ethane.
In any event, the synthesis gas is oxidized in an electrochemical reaction in the anode with oxygen ions received from the cathode via migration through the electrolyte. The reaction creates water vapor and electricity in the form of free electrons in the anode that are used to power the electrical load. The oxygen ions are created via a reduction of the cathode oxidant using the electrons returning from the electrical load into the cathode.
Once the fuel cell is started, internal processes maintain the required temperature for operation. However, in order to start the fuel cell, the primary fuel cell system components must be heated, and some fuel cell system components must be protected from damage during the startup. For example, the anode may be subjected to oxidation damage in the presence of oxygen at temperatures below the normal operating temperature in the absence of the synthesis gas. Also, the reformer may require a specific chemistry in addition to heat, in order to start the catalytic reactions that generate the synthesis gas. Further, the startup of the fuel cell system should be accomplished in a safe manner, e.g., so as to prevent a flammable mixture from forming during the starting process. Still further, it is desirable to purge the fuel cell with a non-explosive and non-oxidizing gas during the initial stage of startup.
What is needed in the art is an improved apparatus and method for startup and shutdown of a fuel cell.